The present disclosure relates generally to multicast network convergence following a failure at a designated router or designated forwarder.
Traditional Internet Protocol (IP) communication allows a host to send packets to a single host (unicast transmission) or to all hosts (broadcast transmission). To support a demand to provide applications that involve high data rate transmission to multiple hosts, a third routing technique has evolved, multicast routing. Multicast routing is used to send data communications to multiple receivers on a network utilizing a multicast address. Multicast routing protocols have been developed to conserve bandwidth by minimizing duplication of packets.
One protocol that supports IP multicast is Protocol Independent Multicast (PIM). A multicast distribution tree is created to ensure distribution to intended receivers while limiting distribution so that network segments that are not in the path between a source and receivers are not burdened with unnecessary traffic. The distribution tree for each multicast group is rooted at a source or a multicast router called the Rendezvous Point (RP). A designated router (DR) sends control messages on behalf of directly connected sources and directly connected receivers for ASM (Any Source Multicast) groups. For bidirectional groups, a designated forwarder (DF) sends control messages on behalf of a directly connected receiver and forwards data packets on behalf of a directly connected source.
On last-hop multicast networks served by redundant routers, an election process for the designated router or designated forwarder (DR/DF) selects one router to send join messages upstream toward the source or RP. If the elected DR/DF fails, a new DR/DF needs to be elected. The new DR/DF then needs to send PIM join messages to build a multicast distribution tree and attract traffic to be forwarded onto the receiver network. This results in multicast network convergence times that increase significantly with increased multicast state in the network. Also, the time it takes for the elected DR/DF to be detected as down, affects convergence time to rejoin on the distribution tree.